Prior art fluoroscopic imaging systems use x-ray tubes that are intermittently powered by application of pulses thereto, which tubes generate the x-ray beams directed through the inwards of a living creature being observed to a screen of phosphorescent material or to an array of scintillators. The pattern of x-ray penetration through the creature being observed is thereby converted to a light pattern. Optics focus the light pattern to generate an image within a video camera. Each pulse of x-ray beams generates a respective image for the video camera, which image is scanned by the electronic camera device to generate a respective frame of video input signal for a video monitor. A video monitor is like a television receiver without the front end portions used to detect video signal from radio waves. The video monitor requires a prescribed regular frame rate for the video signals it receives if, as is the usual practice, the video monitor uses a kinescope with electromagnetic deflection and the electromagnetic deflection uses resonant energy recovery methods. The pulse rate for gating the x-ray tube into operation has been chosen high enough that the regular rate of frames generated by the electronic camera device is sufficiently high to avoid excessive flicker in the average brightness level of the visual display generated by the video monitor. The flicker in the average brightness level can be reduced by using longer-persistence phosphors in the viewing screen of the video monitor, but smear of moving objects becomes noticeable with long-persistence phosphors. The human observer is more sensitive to flicker at higher light levels, but when using normal-persistence kinescope phosphors a 60 Hz flicker rate is deemed to be unobjectionable at normal room light levels when viewing broadcast television (which is raster scanned interlacing the scan lines of alternate fields). Somewhat more display flicker is tolerated in fluoroscopy than in broadcast television, progressive scanning at 30 Hz frame rate being a customary practice in fluoroscopy employing a video camera. The display frame rate is a fixed rate to accommodate resonant energy recovery in the electromagnetic deflection circuitry for the kinescope in the video monitor.
It is known in the prior art that the introduction of a frame storage device between the electronic camera device and the video monitor can avoid x-ray exposures having to be made at the display frame rate in order that the electronic camera device can generate consecutive frames of image samples for the video monitor at a display frame rate high enough to avoid flicker in the average brightness level of the video monitor. Each frame of video camera samples generated during a respective one of less frequent x-ray exposures is written to the frame store in a procedure called "frame grabbing". The writing of the frame store is performed in synchronism with the raster scanning of the video camera, during which frame the image samples are supplied directly to the video monitor. The frame of image samples stored in the frame store can then be repeatedly raster scanned to supply repeated frames of image samples until an updating frame of image samples is available from the video camera responsive to the next x-ray exposure. Accordingly, the use of the frame store facilitates lowering on average the pulse rate for gating the x-ray tube into operation. This reduces the dosage of x-rays to which the living creature being observed is exposed over a given period of time. Because the frame store permits the display to be refreshed at frequent intervals despite x-ray exposures occurring less frequently during a "fluoroscope" mode of operation, shorter-persistence phosphors can be used in the viewing screen of the video monitor. The shorter-persistence phosphors avoid smearing during an alternative, "cinematic" or "cine" mode of operation in which the x-ray exposures occur more frequently--e.g., at the frame rate for the display.
With the use of the frame store, the intolerance of a human observer to motion flicker, or the stroboscopic effect of reduced frame rate, in the video display on the video monitor screen sets a lower limit on the pulse rate for gating the x-ray tube. Experienced clinicians accept considerable amounts of motion flicker when fluoroscopically monitoring certain invasive surgical procedures, such as balloon angioplasty; and the pulse rate for gating the x-ray tube operation can be regular, but slower than a 30 Hz frame rate at which a display is refreshed from a frame storage device.
Motion flicker becomes more noticeable as motion becomes faster, so the degree of reduction of the pulse rate for gating the x-ray tube operation is limited as a function of the rate of motion in the portion of the inwards of the living creature being fluoroscoped. Accordingly, for implementing their methods of x-ray dose reduction, Yassa, Abdel-Malek, Bloomer and Srinivas advocated using fluoroscopic apparatus that provides a range of pulse rates that can be keyed to the nature of the organ(s) being observed by their fluoroscopic system.
The Yassa, Abdel-Malek, Bloomer and Srinivas methods for fluoroscopically observing a living creature with reduced x-ray dosage reduce the frequency at which successive x-ray images are obtained, to be closer to the minimum rate at which motion is satisfactorily observable, and use frame filling apparatus to provide a video monitor with video information at a frame rate sufficiently high that excessive flicker of the average brightness of the display is avoided. The reduction in the frame rate is done, taking into consideration both clinical needs and the speeds of motion of the organs being observed fluoroscopically. Mere observation of organ motion for diagnostic reasons may not have the speed of response requirements that observation of catheter insertion has, for example. For the observation of relatively slow-moving processes--such as peristalsis, for example--a lower frame rate may be selected than for relatively fast-moving processes--such as esophageal action, for example.
In fluoroscopic imaging, the x-radiation is not focussed (although it is fairly well collimated). Accordingly, it has not been the practice in fluoroscopy to select only small portions of the image field associated with high rates of non-uniform motion for more frequent sampling than other portions of the image field exhibiting slower and more uniform rates of motion.
However, a reduction in the frame rate may be done in such manner that the frame rate is no longer uniform, but rather is more reduced during periods of lesser sample interest and is less reduced during periods of greater sample interest. The periods of greater and lesser sample interest are determined in certain of the methods described in U.S. patent application Ser. No. 07/651,074, by analyzing the degree of change in successive x-ray images and increasing image rate when motion between frames becomes excessive. However, the reaction time needed for making such changes is too slow for certain types of fluoroscopic observations. Accordingly, in other methods described in U.S. patent application Ser. No. 07/651,074, the periods of greater and lesser sample interest are determined by recourse to auxiliary organ monitoring means--e.g., an electrocardiograph in the instance where the organ of interest is the head or another portion of the circulatory system.
The Yassa, Abdel-Malek, Bloomer and Srinivas methods rely on frame filling apparatus to reduce the motion flicker that occurs when the pulse rates for paring the x-ray tube are lowered, so that a greater reduction of the pulse rate for paring the x-ray tube can be tolerated and the average dose of radiation over a given time can be reduced still more. The frame-filling apparatus described in U.S. patent application Ser. No. 07/651,074 performs interpolation between successive frames of actual image data, thereby to generate intervening fill frames. In this regard, A. Abdel-Malek, O. Hasekioglu and John Bloomer describe a feasibility study they made with regard to reducing pulse rates for gating the x-ray tube in fluoroscopic apparatus in their paper "Image segmentation via motion vector estimates", SPIE Vol. 1233 Medical Imaging IV: Image Processing (1990), pp. 366-371, published 6 Feb. 1990. This feasibility study (incorporated herein by reference) found that acceleration-compensated interpolation techniques provided superior estimates of fill frames. The feasibility study was made processing sequences of recorded images in extended time. Arranging for frame-interpolation techniques to reduce the motion flicker that occurs when the pulse rates for gating the x-ray tube are lowered, which techniques meet the requirements that they be executed in substantially real time, presented a substantial technical challenge.
Frame-interpolation techniques which are executed in substantially real time are adequate for mere observational procedures, as used in the diagnosis of certain functional abnormalities in the organs of a living creature being subjected to fluoroscopic examination. As the number of fill frames between frames of actual image data increases, however, there is an increased waiting time for the most recent frame of actual image data to occur. This lengthens the response time of the imager, since the display of the most recent frame of actual image data takes place only after the display of all the preceding fill frames generated by interpolation, and since the display of all the preceding fill frames generated by interpolation has to await the generation of those fill frames proceeding from the most recent frame of actual image data. In certain surgical procedures in which the surgeon fluoroscopically observes the results of his actions, the response time of a fluoroscopic imager using interframe interpolation procedures to insert several fill frames between frames of actual image data is adjudged as being too slow to provide the surgeon adequate visual feedback to guide his actions with sufficient precision.
Accordingly, the practice has been to use the "fluoroscope" rather than "cine" mode of operation for monitoring invasive surgical procedures, such as balloon angioplasty in the "cine" mode of operation the frame available for observation is generated by temporally interpolating between the last frame available and its predecessor frame, and time lag in the generation of the frame available for observation becomes very noticeable. In the "fluoroscope" mode of operation the last frame available is continually the one available for observation, and time lag is not as noticable. It is desirable to have fluoroscopic imaging apparatus that provides still another mode of operation, which has both the lower motion flicker of the "cine" mode of operation and the less noticable time lag of the "fluoroscopic" mode of operation.
The inventors have found that, as the ratio of fill frames to frames of actual image data is reduced in order to improve imager response time, it is possible to keep within tolerable bounds the errors associated with extrapolating fill frames from the most recent frame of actual image data using the pixel flow rate and its temporal derivatives as determined on an interframe basis. The generation of fill frames by extrapolation procedures can go forward immediately after receiving the most recent frame of actual image data. Accordingly, imager response time is improved over what is possible with interframe interpolation and the desired new mode of operation can be provided.
The determination of the pixel flow rate and its temporal derivatives on an interframe basis has satisfactory accuracy only if the dosage level for each frame are sufficiently high to keep speckle effects reasonably low, however, so the dosage level for each frame has to be higher than in the "fluoroscopic" mode of operation. The monitoring of certain invasive surgical procedures where a surgeon can tolerate motion flicker, such as balloon angioplasty, may continue better to be done in the "fluoroscopic" mode of operation to reduce patient exposure to x-radiation. Other invasive surgical procedures where a surgeon cannot tolerate motion flicker as well, such as new angioplasty techniques using a whirling cutter to clear blockages from blood vessels, can benefit from the new mode of operation being provided by the fluoroscopic apparatus.